As is commonly known, an optical storage disc comprises at least one track, either in the form of a continuous spiral or in the form of multiple concentric circles, of storage space where information may be stored. Optical discs may be of the read-only type, where information is recorded during manufacture, which data can only be read by a user. The optical storage disc may also be of the type that can be written to, where information may be stored by a user.
For writing information in the storage space of the optical storage disc, or for reading information from the disc, an optical disc drive comprises, on the one hand, rotating means for receiving and rotating an optical disc, and on the other hand optical means for scanning the storage track with an optical beam. Since the technology of optical discs in general, the way in which information can be stored in an optical disc, and the way in which optical data can be read from an optical disc, is commonly known, it is not necessary here to describe this technology in detail.
In all cases, however, to read out or record data, it is necessary to position an optical spot onto the disc track. The position of the readout spot is determined by the position of an objective lens provided for this purpose. Positioning of the readout spot and, therefore, the objective lens should be done in two directions: focus (from and towards the disc) and in a radial direction. This is achieved by moving the objective lens. Thus, the objective lens is mounted in an actuator, and control of the actuator is therefore used to perform focus and radial positioning of the optical spot. The focus position is kept in the plane of the information layer of an optical disc by means of a focus servo system that controls the axial position of the objective lens used for focusing the optical spot, and a radial servo system is provided to control the transverse position of the focus, in order to keep the optical spot focused on the track being scanned.
In principle, an optical disc should be kept in a flat disc shape when it is set in a disc motor, so that an optical pickup unit can keep its optical axis perpendicular to the recording surface of the disc during recording and reproducing operations. During scanning of the recording tracks, the optical pickup unit moves in a radial direction in alignment with the radius of the optical disc.
However, the optical disc set in the disc motor is not flat, mainly due to the manufacturing process. The optical disc curves in both the radial and circumferential directions. As a result, the optical axis of the optical pickup unit cannot be maintained precisely perpendicular to the recording surface of the disc without intervention. The angle formed between the optical axis and the recording surface in the radial direction is defined as the radial tilt angle.
The user data recorded on the optical disc is extracted from the High Frequency (HF) signal. Due to, for example, timing errors in the HF signal, a certain amount of jitter is always present when reading out an optical disc. Some contributors to such jitter are intersymbol interference, crosstalk between neighbouring tracks, disc manufacturing imperfections, together with ordinary noise which is present in all electrical circuits. The tilt angle between the disc and the objective lens results from two principal contributors, namely the disc (manufacturing tolerances and environmental changes) and the drive (objective lens actuator, turntable motor adjustment, axis adjustment, etc). The resulting angular deviations lead to comatic aberrations, i.e. a distortion of the optical readout spot on the disc. This distorted readout spot results directly in a distorted HF signal and, therefore, in timing errors, i.e. jitter. Generally, the jitter increases at a greater rate as the radial tilt becomes larger.
Tighter system tolerances in systems like DVD, DVD+RW and Blu-ray disc (BD) require decreased maximum allowed tilt errors. These maximum allowed tilt errors are specified in a so-called tilt window expressed in tilt window width. For CD, this window is typically +/−15 mrad, whereas for DVD+R/RW it is more typically +/−9 mrad. This tilt window is defined to achieve a jitter below a certain required level (typically 15%). If the total tilt in the readout system is larger than this window, the jitter will be too high and readout of user data is no longer possible.
Thus, as optical recording systems become more and more sensitive to tilt, i.e. angular deviations between the objective lens and the disc, systems (such as recordable DVD or BD) are equipped with means to actively compensate tilt.
There are many known ways of implementing active tilt compensation. Currently, the most common way is to add another degree of freedom to the well-known electro-magnetic two-dimensional (2D) actuator used in CD systems. As a result, a three-dimensional (3D) actuator capable of controlling three degrees of freedom is used. These degrees of freedom are z=focus (toward the disc), x=radial (from inner radius to outer radius) and β=tilt (rotation about the y axis), and this type of tilt compensation mechanism is employed in the arrangement described in International Patent Application No. WO 03/083850. This patent describes a method and device for performing tilt correction using a multi-dimensional actuator, in which a radial tilt value is determined based on a differentiation of focus control values obtained at different radii of the optical disc, and in which two split focus coils and a radial coil are employed, each of which generates a respective force Ff1, Ff2, and Fr depending on the current flowing in the coil windings, whereby an actuator tilt β is generated if Ff1=−Ff2 and a vertical (focus) movement is generated if Ff1=Ff2.
An ideal actuator is completely decoupled, i.e. if a focus control voltage is applied, motion should only occur in the focus direction. Since this type of actuator is built as a mass-spring system, some dynamic behaviour is present and, as a result, the frequency response function of an ideal actuator is equal to a simple 1 degree of freedom (DOF) mass-spring damper system, as illustrated schematically in FIGS. 1a and 1b. However, if the actuator is considered as a rigid body with 6 DOFs suspended in springs, there are 6 eigen-modes. In practice most of these modes show up in all transfer functions. In addition, crosstalk will be present to some extent.
Referring now to FIG. 2a, which is a schematic illustration of the general structure of a conventional tilt compensation mechanism for a 3D actuator, the concept of crosstalk from radial to tilt will now be described in more detail. It will be appreciated that the same principle holds for focus to tilt crosstalk.
A conventional tilt compensation mechanism comprises a radial Proportional Integral Derivative (PID) control unit 10 known as such by a skilled person, a focus PID control unit 20 known as such by a skilled person, and a tilt control unit 30 known as such by a skilled person.
The radial control unit 10 generates a radial control signal r which is amplified by the combined DAC/driver endstage gain Gradial to a voltage Uradial. The radial control signal Uradial is applied to the radial coil of a 3D actuator 40, and as a result, the actuator is caused to move in a radial direction, defined by the transfer function Hradial—to—radial. As explained above, the optical disc is not ideal (i.e. not perfect) and, therefore there will be some radial movement thereof relative to the actuator 40. The actuator 40 can be used to track this unknown disturbance Xdisc. The position of the disc is not known in the drive, so an error signal εradial (corresponding to the difference between the radial position Xradial of the disc and the actuator along the radial direction) is generated and fed back to the radial PID control unit 10 and controlled to zero, with the result that the disc can be tracked.
The focus control unit 20 generates a focus control signal f, which is amplified by the combined DAC/driver endstage gain Gfocus to a voltage Ufocus. The focus control signal Ufocus is applied to the focus coil of a 3D actuator 40, and as a result, the actuator is caused to move in a focus direction, defined by the transfer function Hfocus—to—focus. As explained above, the optical disc is not flat and, therefore there will be some vertical movement thereof relative to the actuator 40. The actuator 40 can be used to track this unknown disturbance Zdisc. The position of the disc is not known in the drive, so an error signal εfocus (corresponding to the difference between the position Zfocus of the disc and the actuator along the focus direction) is generated and fed back to the focus PID control unit 20 and controlled to zero, with the result that the disc can be tracked.
The tilt control unit 30 generates a tilt control signal t which is amplified by the combined DAC/driver endstage gain Gtilt to a voltage Utilt. The tilt control signal Utilt is applied to the tilt coils of the actuator 40, and as a result, the actuator is caused to move in tilt direction, defined by the frequency response function Htilt—to—tilt. An example of such a transfer function is shown in FIG. 2d. 
Crosstalk from radial to tilt is inevitably present to a certain extent, and occurs as a consequence of bad alignment or due to magnet inhomogenities. At low frequency, alignment with the ‘centre of stiffness’ is important, so the force has to act symmetrically in between the suspension springs of the actuator 40. At higher frequencies, the alignment with respect to the centre of gravity (COG) is important, and the force should act precisely through the COG. However, in a 3D actuator, the radial (and focus) force is never acting precisely in the COG and ‘centre of stiffness’, basically due to three main reasons:                manufacturing tolerances,        design limitations,        changing position of the force with respect to the moving part as the moving part has an offset in the focus or radial position. As a result, there is always crosstalk (represented by transfer function Hradial—to—tilt) to a certain extent.        
This concept is illustrated graphically in FIG. 2b, in which the frequency response function (FRF) Hradial—to—radial of an actuator clearly demonstrating this problem is provided. The radial eigen-frequency is clearly visible at 55 Hz. Suppose the disc rotational speed is 100 Hz and the disc has an eccentricity of 0.1 mm. Because the sensitivity at 100 Hz is 0.4 mm/V, a harmonic voltage with amplitude of 0.25 V is needed. The radial control loop (a PID) will generate this control signal Uradial. It will be appreciated that higher harmonics are also generated at multiples of 100 Hz.
In FIG. 2c, a frequency response function Hradial—to—tilt representative of crosstalk from Uradial—to—tilt is illustrated. The gain at 100 Hz equals 65 mrad/V, so the radial voltage of 0.25 V will lead to a tilt amplitude of 0.25*65=16 mrad at a frequency of 100 Hz. Note that the peak at this frequency is due to the torsion eigen-frequency, which is 100 Hz for this actuator. Thus, at 100 Hz disc rotational speed, a harmonic tilt disturbance of 16 mrad is added. Because the bandwidth of current tilt compensation methods is very low (of the order of a few Hz), this tilt is not compensated at all, and the above-mentioned 16 mrad is much larger than the available tilt window. As a consequence, both read and write quality are unacceptably adversely affected.
Crosstalk from focus to tilt is equally inevitable, and the above-described principle applies.
Thus, radial to tilt crosstalk, represented by the frequency response function Hradial—to—tilt in FIG. 2a has an effect on the movement βactuator of the actuator in the tilt direction. Similarly, focus to tilt crosstalk represented by the frequency response function Hfocus—to—tilt in FIG. 2a has an effect on βactuator. These two contributions can be added up in the ‘vector domain’, as illustrated.